The invention relates to a method for power control of a fuel cell system in a vehicle.
A vehicle with a fuel cell system, which is frequently also designated as a fuel cell vehicle, usually consists of a fuel cell system as an energy cell, an electrical energy storage device, generally in the form of a battery, as well as an electrical drive unit which comprises at least one electric motor. In the drive unit the electric motor takes up electrical power and is typically operated as a generator during braking of the vehicle and in this so-called recuperation operation feeds back electrical power, which can then be stored in the electrical energy storage device.
In order to be able to drive such a fuel cell vehicle in the desired manner, typically a torque necessary for the associated required acceleration and accordingly a specific electrical power is determined by means of a driver input, for example the position of an accelerator pedal. In this case for the energy management of the drive system it is necessary for the requested electrical power to be provided by the fuel cell system with sufficient precision and in reproducible behavior, if possible within predetermined dynamics, and sufficiently quickly overall. For the energy management of the drive system, substantial overshoots or undershoots of the power of the fuel cell system are not acceptable. Moreover, by means of the power control it must be ensured that the fuel cell is operated in the allowed power range and a sufficient supply of media for precisely this power range is guaranteed.
Various control methods are known from the prior art. Thus, for example, according to the general prior art it is provided that the power required by the drive system is drawn immediately from the fuel cell or the fuel cell system. Since, typically, in particular in the event of a sudden increase in the power required, the media have not yet adapted to the new working point or, because of the typically necessary flow path, have not yet reached the fuel cell, constitutes a disadvantage. The media supply can be reset with reference to a measurement of the current of the load, but this leads to an extreme loading of the fuel cell, on the one hand, and to extraordinarily poor dynamics of a fuel cell vehicle operated in such a way, on the other hand.
An alternative is described in DE 11 2008 000 986 T5. In this document the structure is not controlled by means of the current but by means of the voltage. The problem with the media supply also arises here, and the dynamics is correspondingly restricted. A further possibility in which the focus is on optimization of the overall efficiency is described, for example, in EP 2 001 070 B 1. However, with this structure, due to the optimization of the efficiency, the dynamics which is crucial in particular for vehicle drives is likewise not very good, which in this case constitutes a serious disadvantage.
Such a structure is known for example from US 2011/0217608 A1. In the variant described there the ultimate objective is a limitation of the maximum current, which is comparatively innocuous at least for the operation of the fuel cell and does not load the fuel cell more than is necessary. However, here too the media supply is not taken into consideration or is considered only indirectly, so that this structure also suffers from the disadvantages referred to above, in particular comparatively poor dynamics, which is, however, highly relevant for vehicle applications.
The object of the present invention is to provide a method for power control of a fuel cell system in a vehicle, which advantageously modifies the method known from the prior art, and which in particular enables a very good and reproducible control behavior with high power dynamics.
The method according to the invention for power control of a fuel cell system in a vehicle, unlike the method according to the prior art, considers a power request made of the fuel cell which, in addition to the requested fuel cell system power by the vehicle, also takes into consideration the power of auxiliary drives of the fuel cell system required in the expected working point. By taking the power required by the auxiliary drives—such as, in particular the air compressor—into consideration in this way, the power request can be implemented very precisely and highly dynamically. In this case the electrical loading of the fuel cell with current is performed in accordance with a model of the cathode dynamics in such a way that the control variable of the control operation is matched to the media dynamics. As a result it is ensured that, as dynamically as possible, the power required from the fuel cell is always only the power which can also be supplied by the fuel cell on the basis of the dynamics of the media supply predicted in accordance with a model. As a result inaccuracies in the control, such as for example undershoots or overshoots, can be largely avoided, since at the time of the power release the fuel cell is capable of supplying the required power, so that a high control quality and a very reproducible behavior is achieved. In this case everything functions with a very high fuel cell dynamics, specifically with the highest fuel cell dynamics that is possible without overloading the fuel cell and thus ultimately both damaging the fuel cell and also reducing the control quality.
With a power interface between the vehicle and the fuel cell system with good control quality, the setup enables good and high accuracy of the provision of power. At the same time the maximum conceivable dynamics, which is only in practice by the available dynamics of the media supply, can be produced.
In this case a very advantageous further embodiment of the method according to the invention provides that the power to be expected from auxiliary drives of the fuel cell system is determined by means of a characteristic field as a function of at least one of the variables power, pressure and/or temperature. Such a characteristic field, preferably a characteristic field in which the expected power of the auxiliary drives can be determined as a function of all three aforementioned variables, is a particularly simple and efficient type for predicting the power of the auxiliary drives to be expected in the target point of the power control. It can then be added very simply by computation to the fuel cell system power requested by the vehicle, in order to receive the actual expected power request made to the fuel cell system in the target region of the power control.
Since specific parameters in this above-mentioned characteristic field can typically be varied by means of the duration of operation of the fuel cell system or of the vehicle, for example through ageing effects, through wear, through leakages or the like which occur, according to a particularly advantageous further embodiment of the idea, the characteristic field can be re-adapted in the static operation of the fuel cell system in order to, even over a relatively long period of operation of the fuel cell system, obtain the best possible prediction of the expected power of the auxiliary drives and thus to maintain a high quality of control.
In the case of the request for the media supply, according to an advantageous further embodiment of the idea underlying the invention, the pressure as well as the mass flow or volume flow of the respective medium is predetermined. As a result, in the method according to the invention the media can be provided as precisely as possible in the desired manner with correspondingly high dynamics.
A further very advantageous embodiment of the method according to the invention also provides that the request for the media supply takes place on the basis of a current setpoint of the power request made of the fuel cell. Such a current setpoint is particularly suitable for controlling or pre-controlling the media supply. In particular in the case of the “cold” combustion in the fuel cell the quantity of substance in the media supply and the electrical current are directly linked. Thus a simple and efficient control of the media supply can be implemented by means of the current setpoint which is very easy to determine from the power values.
Typically the correlation between the current and the voltage and thus ultimately also between the current and the power in a fuel cell is predetermined by the so-called characteristic or polarization curve of the fuel cell. This is subject to corresponding fluctuations under the respective boundary conditions which can change dynamically. Ultimately, however, it may be used in order to determine the current setpoint as exactly as possible from the power request made of the fuel cell.
In order to reduce the computational complexity when determining the current setpoint from the polarization curve of the fuel cell and to make the method correspondingly simple and quick, according to a very advantageous further embodiment of the idea underlying the invention it may be provided that for this purpose the polarization curve is approximated linearly under at least one predetermined boundary condition. Such a linear approximation of the polarization curve of a fuel cell under at least one boundary condition is described in German application DE 10 2013 021 538.2 (which was not prior published) and can be used here analogously in order very quickly and with sufficiently high accuracy to determine the current setpoint from the polarization curve or by means of the polarization curve from the power request made of the fuel cell. The method necessary for this is described in detail in the aforementioned, not prior published document and therefore does not need to be referred to in detail again.
Furthermore, a further advantageous embodiment of the method according to the invention provides that the air mass flow to the cathode is measured and the current setpoint is limited on the basis of the measured air mass flow. The currently prevailing state can be assessed very well by such a measurement of the air mass flow. A limitation of the current setpoint makes it possible, for example, to react to problems in the region of the air supply, in particular to such problems in which no sufficient air mass flow can be provided by the air compressor, since the working point of the compressor is above the surge limit due to a higher pressure drop than usual, for example due to a blocked air filter or the like. In this case the current setpoint value and thus, according to the advantageous further embodiment described above, the media supply controlled thereby is limited to a maximum value corresponding to the actual air mass.
Moreover, a further very advantageous embodiment of the method according to the invention provides that it is ensured, for example by means of a further controller, that the current setpoint passed to the media supply is adapted to the current value drawn from the fuel cell if the vehicle does not take off the current or does not take it off completely. This may happen, for example, if the vehicle is in recuperation mode or, because of a traffic situation, changes dynamically from an acceleration mode into propulsion mode and therefore the power of the fuel cell system can no longer be taken off. Thus in this case the current setpoint is adapted by means of the power control according to the described advantageous further embodiment.
A further particularly advantageous embodiment of the method according to the invention can additionally provide, for all previously described variants of the method, that the current setpoint transmitted to the media supply and/or a setpoint for the current load of the fuel cell are limited in such a way that the permitted operating range of the fuel cell, starting from the point of maximum power on the polarization curve, is in the direction of higher current intensities or in the direction of lower current intensities, preferably in the direction of lower intensities. Thus the operating range of the fuel cell is correspondingly restricted, typically so that the current intensity may fluctuate between zero and the respective current intensity occurring at the point of maximum power. The curve of the fuel cell power over the current is typically a parabola which is open downwards with the maximum power at the maximum point thereon. With a control it can happen that without the corresponding limitation of the power control or of the operating range of the fuel cell either to the sections on the left or the right of the maximum, two different solutions exist for the requested fuel cell system power. In this case the power control would not be stable, so that the corresponding limitation of the working range of the fuel cell ensures a reliable and stable control in said manner.
Moreover, further advantageous embodiments of the method according to the invention are disclosed by the exemplary embodiment which is described in greater detail below with reference to the drawings.